Fix 'browser' help viewer for Windows #4598

Job	Run time
testinstall - - ubuntu-latest	6m 11s
teststandard - - ubuntu-latest	30m 32s
testmanuals - - ubuntu-latest	9m 42s
testexpect testmockpkg testspecial test-compile testlibgap testkernel - - ubuntu-latest	6m 0s
testpackages testinstall-loadall - - ubuntu-latest	27m 39s
makemanuals testmakeinstall - GAPPREFIX=/tmp/gapprefix CONFIGFLAGS="--prefix=/tmp/gapprefix" NO_COVERAGE=1 - ubuntu-latest	6m 17s
teststandard - ABI=32 CONFIGFLAGS="" - ubuntu-20.04	28m 36s
testinstall - HPCGAP=yes ABI=64 - ubuntu-latest	12m 46s
testmockpkg testinstall - BOOTSTRAP_MINIMAL=yes - macos-latest	9m 2s
testbugfix - CONFIGFLAGS="--enable-memory-checking" - ubuntu-latest	33m 33s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=64 BUILDDIR=out-of-tree CONFIGFLAGS="--enable-valgrind" - ubuntu-latest	5m 28s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=32 BUILDDIR=out-of-tree CONFIGFLAGS="" - ubuntu-20.04	4m 18s
testinstall - JULIA=yes CONFIGFLAGS="--enable-debug --disable-Werror" - ubuntu-20.04	6m 10s
testmockpkg testinstall - - windows-2019	31m 16s
Send Slack notification on status change	4s
	3h 37m 34s

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